In order to increase the product yield of potable water without increasing the already high energy consumption from the desalination process, membrane distillation (MD) can be coupled with the existing RO plant by taking advantage of the waste product of the generator set – waste heat – that is usually disposed. MD being a thermally driven process that uses a very small portion of electrical energy (water circulation at low pressure), can both consume this waste heat as input thermal energy for the process, as well as providing cooling for the genset.
Since MD can operate at much higher salinity then RO, the intended setup is to use the concentrated brine from RO to feed the MD system, which can tolerate salinities up to 150,000 mg/L. This means that the overall potable water recovery of this system could be as high as 75% compared to the usual 50% of the RO alone. Because a greater volume of potable water is being produced with a small increase of electrical consumption (approximately 0,5 kWh/m3 for MD while for RO is 3.5 kWh/m3), the overall energy costs per volume of water produced are decreasing when coupling MD to RO. Additionally, since for the RO system some pre-treatment is already in place to avoid scaling in the membranes, no additional pre-treatment is necessary for the MD system.
To resume, with this better use of thermal energy, and the fact that MD can significantly increase the amount of potable water recovered in the desalination process, makes the coupling of the two technologies a very promising and sustainable approach for water supply in remote locations.
Performance increase of membrane distillation pilot scale modules operating in vacuum-enhanced air-gap configuration
Seawater membrane distillation desalination for potable water provision on remote islands – a case study in Vietnam